Multiple layer limited dielectric recording blank for pulse printing

ABSTRACT

A RECORD BLANK OF LIMITED DIELECTRIC PROPERTIES FOR ELECTROGRAPHIC PULSE PRINTING. THE BLANK HAS AN UNDERLYING FIRST DIELECTRIC COATING AND AN OVERLYING SECOND DIELECTRIC COATING OVER A RELATIVELY CONDUCTIVE PAPER SUBSTRATE, WITH THE SURFACE RESISTIVITY EXHIBITED BY SUCH COATINGS IN THE BLANK RANGING FROM 105 TO 1011 OHMS PER SQUARE. A METHOD OF PREPARING THE BLANK FROM AQUEOUS SYSTEMS OF DIELECTRIC COATING MATERIAL.

9, 1973 N. T. ANDERSON 3,709,728

MULTIPLE LAYER LIMITED DIELECTRIC RECORDING BLANK FOR PULSE PRINTING Filed Sept. 25, 1970 Ni/s T/Jflaefson Mad-W70? M HM Tram/545 United States Patent 3,709,728 MULTIPLE LAYER LIMITED DIELECTRIC RECORDING BLANK FOR PULSE PRINTING Nils T. Anderson, Vancouver, Wash., assignor to Crown Zellerhach Corporation, San Francisco, Calif. Filed Sept. 25, 1970, Ser. No. 75,377 Int. Cl. B44d 1/18 U.S. Cl. 117-218 6 Claims ABSTRACT OF THE DISCLOSURE A record blank of limited dielectric properties for electrographic pulse printing. The blank has an underlying first dielectric coating and an overlying second dielectric coating over a relatively conductive paper substrate, with the surface resistivity exhibited by such coatings in the blank ranging from 10 to ohms per square. A method of preparing the blank from aqueous systems of dielectric coating material.

This invention relates to recording blanks or copy sheets, and more particularly to an improved blank or sheet, usable in the production of prints employing electrographic pulse printing procedures. The invention also embraces a novel method for preparing such recording blanks.

In electrographic pulse printing a multiplicity of electrically charged areas are established on selected regions of a recording blank by applying voltage pulses between electrodes disposed on either side of the blank. In a common form of pulse printer, a printing head is present having multiple equally spaced styli or electrodes. During printing, these are moved relative to the front face of the recording blank, and voltage pulses are applied between selected ones of such electrodes and a back electrode on the opposite face of the blank to produce the electrically charged regions earlier described. An alphanumeric symbol may be made to appear on the blank through the preparing of a multiplicity of charged regions so distributed as to form the outline of a symbol, and through proper development wherein the charged regions are made visually distinctive. Reference is made to U.S. Pats. 3,441,437 and 3,465,359 for further particulars of electrographic pulse printing as presently practiced in the art.

As is recognized in the art, a stylus or electrode in an electrographic pulse printer produces a charged region on a copy sheet through ionization of the gas present in the small air gap that exists between the stylus and the front face of the recording blank. Such ionization produces a multiplicity of electrons, and it is the movement of these electrons that results in the development of the region of charge on the recording blank. In the production of such a charged region, the art has taught the necessity of providing a highly dielectric or insulating film on the recording blank to prevent disruptive breakdown of the ionization process.

This invention is based on the discovery that a copy sheet or recording blank with what I refer to as limited dielectric properties may be prepared, for use in electro graphic pulse printing, which possesses a number of advantages over copy sheets as previously known. The invention further concerns a novel method of preparing such a copy sheet, which results in the production of a highly satisfactory product economically, and in a practical manner well suited for adoption by a paper manufacturer.

Among the objects of the invention, therefore, is the provision of a recording blank for electrographic pulse printing characterized by limited dielectric properties as further described herein.

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A further object is to provide a copy sheet for electrographic pulse printing, which may be prepared with many of the aesthetic qualities associated with conventional type papers. Further explaining, the sheet or blank of the invention is characterized by a very thin spread of dielectric material including opacity producing pigment over the face thereof, with the result that bond or other type papers may be produced usable in pulse printing, which retain the appearance of such papers and do not have the heavily loaded appearance of conventional electrographic printing papers.

Yet another object of the invention is to provide a copy sheet or recording blank which performs highly satisfactorily in producing prints using electrographic pulse printing techniques, which may be relatively easily manufactured from readily available, relatively low cost materials. The copy sheet, therefore, is extremely well suited for the production of alphanumeric information in recorded form where it would be expected to record large volumes of such recorded information.

A copy sheet, while produced from readily available materials at low cost, has excellent nonblocking and triboelectric properties, as well as other characteristics commending the paper for use as a recording medium.

These and other objects of the invention will become more fully apparent from the following description, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates, on a greatly enlarged scale, a cross section of a copy sheet as contemplated by the invention; and

FIG. 2 is an enlarged schematic drawing, illustrating portions of a copy sheet during the process of being charged using an electrographic pulse printer.

As briefly discussed earlier, in electrographic pulse printing, using a bank of styli or electrodes in a printing head, or by other means, distributed regions of charge are produced on a recording blank, which regions in the aggregate and when developed function to display an alphanumeric symbol. The necessity for retaining the charge in the distributed regions has in the past dictated the use of a recording blank surfaced with a highly dielectric or insulating film. This has contributed to the cost of the blank, both by reason of the cost of the materials going into the blank, as well as the nature of the manufacturing steps necessary to produce the paper. Furthermore, the blanks or copy sheets have generally been characterized by a heavily loaded and/or glossy look, detracting from their aesthetic appearance.

As earlier described, the production of a charged region on a recording blank involves ionization of gas in a gap between the printing electrode or stylus and the sheet. As recognized in the art, a so-called threshold voltage is required between the printing electrode or stylus and a backup electrode on the other side of the sheet, which is in the neighborhood of 340 volts, to bring about such ionization.

I have discovered that exceptionally good results can be realized, using voltages ranging between this threshold voltage and about 1100 volts, with copy sheets having limited dielectric properties, i.e., with sheets surfaced with a dielectric material where the dielectric surfacing exhibits in the copy sheet a surface resistivity ranging between 10 and 10 ohms per square. Using the copy sheets contemplated, and with pulse durations of less than 5 milliseconds, preferably of less than microseconds duration, a print may be prepared which quite surprisingly has excellent print resolution and density.

Toward attaining these ends, a copy sheet is contemplated comprising a paper substrate and a dual coating dielectric surfacing over such substrate. The combined weight of the two coatings in such surfacing does not exceed, and is usually less than about 7 pounds per ream,

on a solids basis (one ream being 3,000 square feet). The substrate is made relatively electrically conductive, to permit the fiow of electrons to a region directly adjacent the underlying face of the conductive surfacing in the copy sheet. By relatively conductive, it is contemplated that the substrate exhibit in the final copy sheet a surface resistivity which is in the order of at least one magnitude less than the surface resistivity of the dielectric surfacing.

The surface resistivities referred to herein are resistivities as measured on the final copy sheet or recording blank. Because of the very thin dielectric surfacing contemplated by the invention, when the dielectric surfacing of the copy sheet is measured for surface resistivity, the value measured reflects the fact that there is some leakage of electrons across the dielectric surfacing and through the relatively conductive substrate. The dielectric materials used in preparing the dielectric surfacing, were they measured for volume resistivity using conventional meth ods, would exhibit a resistivity in ohms centimeter of a substantially greater magnitude.

Surface resistivity measurements were performed here in using a modification of ASTM D257-61, wherein the resistance is measured with direct current on a square area of the copy sheet. Specifically, a General Radio type 1230-A electrometer was used, having a probe comprising two parallel copper plates separated by a block of Teflon. The plates, which had a thickness of A; inch, had a contacting length of one inch, and were spaced apart a distance equal to the contacting length of the plates, or a distance of one inch. The unit was shielded to reduce electrical noise. Measurements were made by placing the probe, including the two plates, against the surface being tested, and the resistivity measured was done with a direct current applied between the two plates, the meter being graduated to give directly a reading in ohms per square.

It is felt that superior prints result employing the recording blanks of the invention for several reasons. For one thing, image density to a good approximation is inversely related to dielectric thickness. As a consequence, with the thin dielectric surfaces that characterize the sheets of the invention, optimum image densities are possible. It is also believed that electron emission from the dielectric surfacing is promoted with a surfacing of limited dielectric properties. As a consequence, and considering a pulse printing procedure where it is desired to produce a region of positive electrical charge on the surface of the copy sheet, because of electron emission from the dielectric surfacing, there is greater and faster ionization of the gas in the gap between the print electrode and sheet, with a greater mass of electrodes leaving the surface of the sheet to leave a positively charged region. Whatever the reasons involved, it has been found that prints are producible far exceeding in quality the prints normally to be expected.

According to the invention, the dielectric surfacing is prepared by applying two dielectric coatings to a paper substrate, with the combined coating weight of such coatings being less than about 7 pounds per ream, on a solids basis. Ordinarily, the underlying of these two dielectric coatings is present in a coating weight which is from 50 to 90% of the combined coating weight. It is important that the surfacing be prepared by the application of multiple coatings, to avoid the production of thin spots in the coating, resulting in a dielectric breakdown during pulse printing.

In this connection reference is made to FIG. 1 which shows in cross section a copy sheet prepared with a dual coated dielectric surfacing as contemplated. Illustrated in the figure is a substrate 10, an underlying dielectric coating 12 over one face of this substrate, and an overlying second dielectric coating 14 over coating 12. When a single coating is applied in weights ranging for example from 1 to 4 pounds per ream on a solids basis, it is to be expected that thin regions, exemplified by the depression 18, are produced at random points throughout the coated product. These may be the result of several factors, such as sorption of the vehicle of the coating composition into the paper with there being a lack of pigment and residual binder where such occurred, unevenness produced by the method of application, etc. With the second coating applied over the underlying coating, a dielectric surfacing is produced which is free of such depressions, the second coating filling up the thin regions present in the first coating. As a consequence, even though a relatively thin overall coating weight is employed, a dielectric surfacing of limited surface resistivity is produced which is not subject to dielectric breakdown in pulse printing.

Any number of dielectric materials might suggest themselves for the production of the dielectric surfacing in the recording blank or copy sheet. The resins function as a binder in the coated product, adhering to any substrate over which the material may be applied, and holding together particles of pigment that are normally present. As recognized in the art, dielectric resins which may be employed comprise both solvent carried resin solutions, as well as emulsions, the latter being typified by resin latices which are emulsions of resin in a water vehicle. Illustrative of solvent carried resin solutions are such homopolymers and copolymers as polyvinyl acetate or acid copolymers thereof such as a crotonic copolymer, shellac, polyvinyl chloride, acrylates, alkyds, polyvinyl butyral, polyesters, etc. Illustrative of common resin latices are such materials as styrene-butadiene copolymers and terpolymers such as styrene-butadiene-acrylonitrile copolymers, vinylidene chloride resins, butadiene acrylonitrile copolymers, etc.

It is preferred in practicing the instant invention to employ as a resin in the outer dielectric coating a resin having proper triboelectric properties coupled with nonblocking characteristics, low gloss, etc. In this way the printing performance of the copy sheet is optimized. In this connection several resins suggest themselves, such as carboxylated polyvinyl acetate and polyvinyl butyral. Of these, carboxylated polyvinyl acetate is preferred, as such is soluble in an ammoniacal aqueous solution (pH ranging from 7.5 to 10), and a spread prepared from such a solution is curable to render it nonsolu'ble, i.e., resistant to water, by heating the spread to drive off ammonia present.

The coating compositions employed, in addition to resin, include an inert, opacity producing pigment, or pigments, such as barium sulphate, titanium dioxide, nonphotoconductive zinc oxide, clays, zinc sulphide, and other mineral pigments. -By inert, it is intended to refer to pigments that are not photoconductive, and do not have conductive imparting properties. Such pigments are present in proportions (related to hinder) ranging from about 0.25:1 to 2.5 :1. The amount of pigment present in a coating composition is not critical, the lower limit being dictated by economic reasons, the amount of pigment needed to obtain proper opacity, texture, etc., and the upper limit by the need of obtaining a coated product which is not crumbly and subjected to cracking, and which retains its flexibility.

As will be appreciated by those skilled in the art, in addition to the resin and pigment, the coating compositions may include minor amounts of other ingredients, present to modify the processing characteristics of the paper in its manufacture, or the physical properties of the finished product. Thus, the coating compositions may include materials such as waxes, silicones and stearate salts to reduce tack and improve spreading characteristics. Ordinarily, such additives collectively comprise no more than about 5% of the resin solids in a coating composition.

In selecting the composition used for the underlying dielectric coating, blocking and triboelectric properties are not a principal consideration because such is masked by the overlying dielectric coating in the dielectric surfacing. However, factors such as the cost of the coating composition, the manner and ease in which it may be applied, etc. enter into the selection of the resin used. In this connection, it has been found that a coating composition including as binder a butadiene-styrene copolymer is eminently satisfactory. The mixed polymer is a good dielectric material, that bonds well to a paper substrate. The polymer may be applied as a latex or water emulsion, and is thus readily handled in a paper-making plant. Additionally, the resin material is relatively inexpensive making it attractive for the production of recording or copy sheets intended to be used in high volume.

As described earlier, the substrate in the recording blank or sheet of the invention should have a surface resistivity, exhibited in the final product, which is at least one order of magnitude less than the surface resistivity of the dielectric surfacing on the front face of the sheet. This relative conductivity in the substrate may be obtained in a number of ways, but in a preferred embodiment the conductivity is obtained by impregnating the paper web with a water soluble conductive salt. Exemplary of such salts are the following, all of which have solubility in water exceeding one part per 100 parts of water: ammonium nitrate, calcium chloride, sodium nitrate, sodium chloride, etc.

The water soluble conductive salt preferably is introduced into the substrate after the preparation over the paper web of the dielectric surfacing discussed above. The dielectric coatings in the surfacing are prepared in such a manner that after distribution over the substrate they are rendered resistant to water. This permits a subsequently applied aqueous solution of conductive salt to be spread over the paper substrate without such disrupting already applied dielectric coatings. By applying the dielectric coatings before the substrate is rendered conductive, contamination of the dielectric coating compositions with conductive salt is prevented. In this connection, it should be remembered that when a coating is applied usually such is applied with flooding, and excess is removed as with an air knife, and where a water based dielectric coating composition is used, were water soluble material in the substrate, such would be removed with the excess to produce contamination. By applying the conductive salt in this manner, the additional advantage is realized in that the moisture content of the copy sheet may be controlled.

For the purpose of imparting to the underlying dielectric coating greater resistance to water, an insolubilizer may be included in the coating composition. For instance, with a water carried butadiene-styrene latex composition, when such is applied the latex is insoluble in water but dispersible by water. A film of such material, upon aging, converts to a material which is resistant to water. The addition of an insolubilizer, however, accelerates this conversion. Such insolubilizer may take the form of a water soluble, melamine formaldehyde resin. The amount of such resin included with the butadiene-styrene composition ordinarily comprises no more than about 20% by weight of the mixed polymer. With such present in the coating composition, and on the coating. composition having been spread and with subsequent heating, the coating is rapidly converted into a water resistant state. Ordinarily, temperatures between about 160 F. and temperatures at which charring or other damage might occur (around 300 F.) are used, with curing times ranging about one to about minutes.

Further illustrating a specific embodiment of the invention, a 35 pound per ream bond paper was first coated with a dielectric coating composition comprising an aqueous dispersion of a butadiene-styrene mixed polymer. The polymer contained about 60% styrene and 40% butadiene, and is available as Latex 636 from Dow Chemical Company. Also included in the coating composition was an inert pigment, in the form of a mixture of barium sulphate and zinc sulphide, such being available commercially from Sherwin-Williams as Lithopone Permolith 60H. In addition to the butadiene-styrene mixed polymer the composition contained an insolubilizer in the form of a soluble melamine formaldehyde resin. The coating composition contained, for each parts butadiene-styrene mixed polymer, 2.5 parts of melamine formaldehyde resin, and 50 parts of pigment. The coating composition included sufiicient water to produce a solids content of approximately 50%.

The bond paper was coated with a spread of 4 pounds per ream, on a solids basis. The coating was cured, to render the coating resistant to water, by subjecting it to 5 minutes of heating at 180 F.

A second dielectric coating was then prepared over the underlying dielectric coating, from a coating composition comprising an aqueous solution of carboxylated polyvinyl acetate. The composition contained, for each 100 parts polyvinyl acetate (Gelva C3V20 from Monsanto), 50 parts Lithopone Permolith 60H, and 0.5 part ammonium stearate. The coating composition included sufiicient water to produce a mixture of 50% solids content, and was adjusted to a pH of approximately 9 by the addition of ammonium hydroxide.

The coating composition described was applied using a spread of 1.5 pounds per ream, on a solids basis. The coating was then cured by heating the product at F. for 5 minutes which had the effect of driving off the ammonia and rendering the coating insoluble in water.

As a final treatment the coated paper was coated over its back face with an aqueous solution of sodium nitrate, at 25% concentration. Such was applied by spraying it onto the paper web, at a rate of about 3 pounds per ream (solids basis).

The front dielectrically coated side of the copy sheet so produced was measured for surface resistivity, as was the back or paper side. The front side of the copy sheet exhibited a surface resistivity of 10 ohms per square, and the back side of the copy sheet exhibited a surface resistivity of 10 ohms per square.

The recording blank was then processed to print thereon alphanumeric information, using electrographic short pulse printing procedures.

There are various forms of equipment for electrographic pulse printing available commercially, exemplified by the Printipix Tube produced by Scientific Recorders, the Clevite 4800 made by Gould, the Info-Max, and the computer oriented electrostatic printer of Versatec, Inc. As represented by the schematic illustration which forms FIG. 2 in the drawings, such a printer may include a bank of styli or electrodes as represented by electrode 20, which form the printing head in the machine. The recording blank 21 is moved across this printing head, while supported on backup support means such as the rolls shown at 22 which comprise a backup electrode to produce charged regions on the sheet. DC voltage pulses are applied between selected ones of the electrodes 20 and the backup support forming an opposing electrode. The voltage applied between opposed electrodes ordinarily ranges between a threshold voltage of about 340 volts and 1100 volts.

Demonstrating the utility of the recording blank of the invention, such was printed in an electrographic pulse printing machine using pulses of 100 microsecond duration. Charged regions on the recording blank were developed using conventional developer material to produce a visible image. Excellent prints resulted, exhibiting good image density and resolution.

It is claimed and desired to secure by Letters Patent:

1. A coated recording blank of limited dielectric prop erties for electrographic pulse printing with pulses of less than 5 millisecond duration, comprising a paper substrate in the form of a paper web impregnated with a water soluble conductive salt, an underlying dielectric coating over one face of said paper substrate which is a mixture of dielectric resin and pigment consisting of inert opacity producing pigment, and an overlying second dielectric coating on the underlying dielectric coating which is also a mixture of dielectric resin and pigment consisting of inert opacity producing pigment, the proportion of pigment to resin in each of such coatings ranging from 0.25:1 to 2.521, said coatings exhibiting in the recording blank a surface resistivity ranging from 10 to ohms per square and having a combined weight which is less than 7 pounds per ream, the under- .lying coating having a coating weight which is from 50 to 90% of the combined coating weight, said substrate exhibiting in the recording blank 3. surface resistivity which is at least about one order of magnitude less than the resistivity exhibited by said coatings in said blank.

2. A coated recording blank of limited dielectric properties for electrographic pulse printing with pulses of less than 5 milliseconds duration, comprising a paper substrate in the form of a paper web impregnated with a water soluble conductive salt, an underlying dielectric coating over one face of said paper substrate prepared from a water-carried mixture of a butadiene-styrene, copolymer and pigment consisting of inert opacity producing pigment, said underlying coating having been cured to a water resistant state, and an overlying dielectric coating on the underlying dielectric coating prepared from a water-carried mixture of a polyvinyl acetate resin and pigment consisting of inert opacity imparting pigment, said overlying coating having been cured to a water resistant state, the proportion of the pigment to resin in each of said coatings ranging from 0.25:1 to :1, said coatings exhibiting in the recording blank a surface resistivity ranging from 10 to 10 ohms per square and having a combined weight which is less than 7 pounds per ream, the underlying coating having a coating weight which is from 50 to 90% of the combined coating weight in the blank, said substrate exhibiting in the recording blank a surface resistivity which is at least one order of magnitude less than the resistivity exhibited by said coatings in said blank.

3. A coated recording blank of limited dielectric properties for electrographic pulse printing using pulses of less than 5 milliseconds duration, comprising a paper substrate, an underlying dielectric coating over one face of said substrate which is a film prepared from a watercarried mixture of dielectric resin and pigment consisting of inert opacity imparting pigment, which film has been cured to a water resistant state, and an overlying dielectric coating on the underlying dielectric coating which is a film also prepared from a water-carried mixture of dielectric resin and pigment consisting of inert opacity imparting pigment, which film has been cured to a water resistant state, said coatings exhibiting in the recording blank a surface resistivity ranging from 10 to 10 ohms per square and having a combined coating weight which is less than 7 pounds per ream, said substrate exhibiting in the recording blank a surface resistivity of at least about one order of magnitude less than the resistivity of said coatings in said recording blanks.

4. The recording blank of claim 3, wherein the paper substrate comprises a paper web impregnated with a water soluble conductive salt.

5. The method of producing a coated recording blank of limited dielectric properties for electrographic pulse printing with pulses of less than 5 milliseconds duration, comprising applying a first coating to one face of a paper web using a mixture of water-carried dielectric resin and pigment consisting of inert opacity producing pigment mixture, curing the coating so produced to render the same resistant to water, applying a second coating over said first coating using a mixture of water-carried dielectric resin and pigment consisting of inert opacity producing pigment, curing the coating so produced to render the same resistant to water, and applying to the opposite face of the paper web an aqueous solution of water soluble conductive salt with such impregnating the web and distributing salt therein to produce a relatively electrically conductive substrate therein, the underlying and overlying dielectric coatings being applied to have a combined coating weight which is less than about 7 pounds per ream and imparting to the recording blank a surface resistivity ranging from 10 to 10 ohms per square, the water soluble conductive salt solution being applied in a quantity whereby the substrate imparts to the recording blank 2. surface resistivity which is at least one order of magnitude less than the resistivity of said coatings in said blank.

6. The method of producing a coated recording blank of limited dielectric properties suitable for electrograp'hic pulse printing with pulses of less than 5 milliseconds duration, comprising applying a first coating to one face of a paper web using a mixture of water-carried butadienestyrene dielectric resin and pigment consisting of inert opacity producing pigment, curing the coating so produced to render the same resistant to Water, applying a second coating over said first coating using a mixture of water-carried polyvinyl acetate dielectric resin and pigment consisting of inert opacity producing pigment, curing the coating so produced to render the same resistant to water, and after the production of such coatings applying to the opposite face of the paper web an aqueous solution of water soluble conductive salt with such impregnating the web and distributing salt therein to produce a relative electrically conductive substrate in the recording blank, the underlying and overlying dielectric coatings being applied to 'have a combined coating weight which is less than about 7 pounds per ream and imparting to the recording blank a surface resistivity ranging from 10 to 10 ohms per square, the water soluble conductive salt being applied in a quantity whereby the substrate imparts to the recording blank a surface resistivity which is at least one order of magnitude less than the resistivity of said coatings in said blank.

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